Fuel cells, oxygen sensors, oxygenation membrane devices, and the like have an electrolyte-electrode joined assembly produced by forming an anode and a cathode on surfaces of an oxide ion-conductive electrolyte. In such an assembly, oxygen is ionized on the cathode, and generated oxide ions are transferred through the electrolyte to the anode.
The above oxide ion transfer is accelerated at a relatively high temperature. The electrolyte-electrode joined assembly must be heated to such a high temperature in order to drive the fuel cell, etc. Thus, the fuel cell cannot produce electric power immediately as needed.
Accordingly, electrolytes containing substances excellent in oxide ion conductivity even at relatively low temperatures, have been proposed in recent years. Specifically, the substances include fluorite-type oxides such as scandium-stabilized zirconias (ScSZ) and samarium-doped cerias (SDC), perovskite-type oxides such as lanthanum gallate (LaSrGaMgO), etc. In addition, the applicant has proposed an oxide ion conductor containing an apatite-type composite oxide (see, e.g. Patent Document 1).
The apatite-type composite oxide is excellent in oxide ion conductivity. However, when the composite oxide is used as the electrolyte in the electrolyte-electrode joined assembly, the assembly often exhibits an insufficient power generation function. As a result of intense research, the present applicant has found that this is because the assembly has a relatively high interface resistance between the electrode and electrolyte, and thereby has a considerable IR loss. Thus, the present applicant has proposed an intermediate layer containing an isotropic conductive material, formed between the electrolyte and electrode in order to achieve a sufficient power generation function, in Patent Document 2.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-252005    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-149795